An old idea is to treat a gas and a granular solid material by causing the gas to flow in the horizontal direction across a bed of the solid material disposed in a “panel” that has often been tall in comparison with its width in the direction of gas flow. Often, the “panel bed” has been held in place by louvered walls that resembled Venetian blinds. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) cites early art and is incorporated by reference in the instant application.
Some designs called for continuous or intermittent motion of granular material downward through the panel, fresh material being supplied at the panel's top and “spent” material being withdrawn at its bottom. A representative recent proposal along this line is to be found in U.S. Pat. No. 5,527,514 (Jun. 18, 1996). U.S. Pat. No. 4,017,278 (Apr. 12, 1977) provided performance data for a gas-filtration device of this kind. The panel contained “gravel” 2 to 12 mm in size. Superficial velocity of dusty gas approaching the panel was 25 to 100 cm/s. Herein, superficial velocity=rate of gas flow divided by the projected vertical frontal area of the panel (panel height×panel width). Downward speed of the gravel mass was 30 cm/hr. Dust content of filtered gas ranged from ˜25 to ˜150 milligrams per normal cubic meter (mg/Nm3). Filtered dust accumulated within the gravel bed, not upon the gas-entry faces that it presented. In other words, the device filtered the dust by what practitioners term a “deep-bed filtration mechanism.”
In other designs, granular material was stationary much of the time. These designs filtered dust from gas by accumulating a cake of the dust (a “filter cake”) upon gas-entry faces of granular material retained in a panel bed. Such designs are capable of providing filtered gas containing levels of residual dust comparable to that provided by fabric filtration. Both a panel bed of this type and a fabric filter employ a “surface filtration mechanism,” in which the filter cake is in fact the filtration medium; the primary function of either granular bed or fabric is to support the cake. Means are provided for intermittent renewal of gas-entry faces through removal of a moiety of the granular material from these faces together with accumulated dust. Means for outwardly tipping louvers that support gas-entry faces and for drawing plows horizontally along the faces have been proposed. U.S. Pat. No. 3,800,508 (Apr. 2, 1974) not only provided means for pivoting louvers but also employed a gas-entry velocity at gas-entry faces sufficient to support the faces at an angle steeper than the dynamic angle of repose of the granular material; momentary interruption of gas flow produced a spill of this material along with filter cake.
My U.S. Pat. No. 3,296,775 (Jan. 10, 1967) disclosed a puff-back method for renewing gas-entry faces of a panel bed of the type wherein louvers support gas-entry portions of the bed. Puff-back entailed creation of a reverse transient surge flow to produce en masse displacement of the bed's gas-entry portions respecting the supporting louvers. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) specified a reverse transient surge flow of a more particular character, whose discovery made possible development of a practical panel-bed filter employing a surface filtration mechanism for cleaning a dusty gas (K. C. Lee, I. Rodon, M. S. Wu, R. Pfeffer, and A. M. Squires, The Panel Bed Filter, EPRI AF-560, Electric Power Research Institute, Palo Alto Calif., May 1977; I. Rodon, K. C. Lee, R. Pfeffer, and A. M. Squires, Panel Bed Filtration Data for Three Dusts at 150° C., paper 79-56.5 presented at meeting of Air Pollution Control Association, Cincinnati, Ohio, June 1979; A. M. Squires, K. C. Lee, and R. Pfeffer, The Panel Bed: A Fluid-Solid Contacting Device Exploiting a New Mode of Soil Failure, paper presented at POWTECH 81, Birmingham, England, March 1981). Operation of a panel-bed filter is cyclic, an interval of filtration alternating with a puff-back that removes both filter cakes and a moiety of sand lying directly beneath the cake, thereby renewing the bed's gas-entry faces. During an interval of filtration, paralleling the formation of the filter cake is an increase in pressure drop in the gas flowing across the bed. This pressure drop cannot be allowed to increase without limit, for two reasons: an unduly large pressure drop would increase cost for gas-compression beyond an economic limit and would impose a force upon the filter cake sufficient to break off chunks of the cake, driving these deep into the granular bed and harming filtering efficiency. Although the object of face-renewal is to present a new free face, it is inadvisable to employ such a strong puff-back as to yield an absolutely clean face. In operation of panel-bed filters, experience has taught that a clean face does not filter as well as a somewhat dirty face. In a subsequent filtering interval, I believe, a new filter cake forms quicker upon a dirty face than upon a strictly clean one.
Commercial-scale panel-bed-filter modules have now successfully cleaned hot gaseous products of combustion of both coal and wood waste, hot gas from cement production, and hot gas from electrometallurgical manufacture of ferrosilicon (this latter gas, containing a fine silica fume, is particularly difficult to clean). Typically, dust in the cleaned gas amounted to less than 5 mg/Nm3.
The tests used ordinary sand as granular medium. Suitably, the sand was about 0.15–0.45 mm in size. In small-scale tests at elevated temperature (e.g., 150 to 500° C.), I found this size, for dusts studied so far, to be substantially the largest sand size upon which a filter cake of good integrity can accumulate. Use of sands of smaller sizes affords filtered gas at lower residual dust remaining, at cost of either lower throughput or higher pressure drop in the filtered gas. For a given size of sand, to allow a filter cake to form, there are limits upon the velocity of gas entering a free face of the sand. For the 0.15–0.45 mm sand, in tests filtering a number of dusts at about 150–200° C., preferred gas-entry velocities have ranged from ˜16 to ˜28 cm/s (superficial velocities, from ˜8 to ˜14 cm/s). Herein, gas-entry velocity=rate of gas flow divided by the total nominal areas of free sand faces upon which a filter cake can accumulate. Since the profile of a gas-entry face, seen in vertical cross-section, is poorly defined, it is convenient to define a nominal area, thus:Gas-entry face area=(straight-line distance between the face's outer and inner edges in the direction perpendicular to the edges)×(the mean of the horizontal lengths of these edges).(Notice that this definition applies, regardless of whether the face's edges are straight or curved in the horizontal direction. In state-of-the-art designs, edges are straight, but louvers that are circular in a plan view may be useful in some applications.) In general, a lower gas-entry velocity is preferable the smaller the size of dust to be filtered or the less cohesive the dust.
Parenthetically, I note that both larger granular material sizes and higher gas-entry velocities may be specified for panel-bed applications wherein a clean gas is treated by a contact with the granular material.
In a test of a commercial-scale panel-bed filter module, wood ash was filtered from gas at 200° C. emitted by a wood-waste boiler (H. Risnes and O. K. Sønju, Evaluation of a novel granular bed filtration system for high temperature applications, paper presented at conference on Progress in Thermal and Biomass Conversion, Tyrol, Austria, 2000). The module comprised two panel beds, each 600 mm in width and 3000 mm in height. For each bed, the projected vertical frontal area was about twice the nominal area of gas-entry faces. Thus, superficial velocity of gas was about one-half the gas-entry velocity. Gas-entry sand faces received and filtered dusty gas supplied to a space surrounding the module. In cooperation with nonporous side panels, the two panel beds enclosed a central space in which cleaned gas moved upward toward an outlet. On average, residual dust in filtered gas was 1.7 mg/Nm3. This excellent filtration performance has been confirmed in tests of an installation comprising 27 substantially identical modules receiving gaseous products of combustion at 190° C. from a 5 Mw wood-waste boiler (the tests having been marred, however, by a poor choice of apparatus for separating puff-backed sand from the filtered wood ash).
At its present stage of development, the panel-bed filter with puff-back is an economically attractive choice for many applications (especially, for example, for removing ash from wood-waste combustion or silica fume arising from FeSi production). An important parameter for judging a device for removing dust from a gas is the area of ground that it occupies. The “footprint” of a state-of-the-art panel-bed filter is approximately 0.13 square meter per 1,000 cubic meters of dusty gas to be filtered per hour. In contrast, the footprint of a high-efficiency electrostatic precipitator is about 0.37 m2/(Km3/hr). A fabric filter's footprint is even larger.
There is, however, need for improvement. The fabricator of modules for tests on a wood-waste boiler, described above, complained at the large number of individual louver elements needing to be assembled. A higher gas-treating capacity per louver would constitute an advance in the art. In addition, cost of filter modules is a major part of the total cost of a filter installation. A larger gas-treating capacity per filter module, reducing the number of modules needed for a given application, would also advance the art.
In a second utility application filed simultaneously with the instant application, entitled IMPROVED PANEL-BED METHOD AND APPARATUS FOR GAS AND GRANULAR MATERIAL CONTACTING, (now U.S. Pat. No. 6.783,572) I disclose an advance in panel-bed art capable of                achieving a four-fold or greater increase in the capacity of a panel bed of given projected vertical frontal area.In a panel bed used as a filter, the disclosure has potential for achieving        a four-fold or greater decrease in the number of individual louvers that must be manufactured and assembled, and        a shrinkage in footprint from the aforementioned 0.13 m2/(Km3/hr) to approximately 0.09 m2/(Km3/hr).The disclosure, however, has the disadvantage that the ascribed increase in capacity and shrinkage in footprint can be achieved only at cost of an increase in pressure drop. Such increase is not a significant disadvantage for panel-bed applications requiring countercurrenticity of contacting of a gas and a granular medium; but where a large flow of a dusty gas must be cleaned by filtration, the increase in pressure drop can be costly. Accordingly, the disclosure of my second utility application of today's date leaves open a place for an arrangement that increases filtration capacity while not incurring this penalty.        